Filtering components which can be incorporated in optical waveguides have been the focus of increasing attention in the last two decades. The outstanding example at the moment is the Bragg grating which is directly written into optical fibers in order to fulfill various filtering functionalities such as laser frequency stabilization, gain equalization in erbium-doped fiber amplifiers, and channel adding or dropping in wavelength multiplexed optical networks.
The essential feature of a Bragg grating is a locally periodic variation in the refractive index profile that is incorporated into the waveguide along the propagation direction. Serious problems arise when one wants to superimpose several such Bragg gratings in order to filter several wavelengths. In the case where Bragg gratings are written into a fiber through the UV holographic technique, one observes that the Bragg gratings interfere with each other, the reflectivity peaks of earlier gratings being displaced by the ones written later. This mutual interference is due in part to the saturation of the UV-induced change in the refractive index and to the DC (or nonsinusoidal) component of the UV-induced change in refractive index.
Another serious drawback of Bragg grating technology in optical fibers is the limitation of the refractive index modulation to the 10−4–10−3 range.
In semiconductor lasers another technique has been used in order to create Bragg gratings, that being to introduce a periodic corrugation in one of the layers constituting the semiconductor laser waveguide. This corrugation modulates in a locally periodic way the effective index profile of the laser waveguide thereby achieving the same effect as modulating the index of refraction in fiber Bragg gratings. The same problems arise with this corrugation technique as with the fiber Bragg technique when one wants to physically superimpose several gratings.